Now that you've seen the details of class inheritance in the previous chapter, it is a good time
to take a step back and look at Scala's class hierarchy as a whole. In Scala, every class inherits
from a common superclass named Any. Because every class is a subclass of Any, the methods
defined in Any are "universal" methods: they may be invoked on any object. Scala also defines
some interesting classes at the bottom of the hierarchy, Null and Nothing, which essentially
act as common subclasses. For example, just as Any is a superclass of every other class,
Nothing is a subclass of every other class. In this chapter, we'll give you a tour of Scala's
class hierarchy.

Because every class inherits from Any, every object in a Scala program can be compared using==, !=, or equals; hashed using hashCode; and
formatted using toString.
The equality and inequality methods, == and !=, are declared final
in class Any, so they cannot be overridden
in subclasses. In fact, == is always the same as equals and !=
is always the negation of equals. So individual classes can tailor what
== or != means by overriding the equals method.
We'll show an example later in this chapter.

Class hierarchy of Scala.

The root class Any has two subclasses: AnyVal and AnyRef.
AnyVal is the parent class of every built-in value class in
Scala. There
are nine such value classes: Byte, Short, Char, Int,
Long, Float, Double, Boolean, and Unit. The first eight
of these correspond to Java's primitive types, and their values are
represented at run time as Java's primitive values. The instances of these
classes are all written as literals in Scala. For example, 42 is
an instance of Int, 'x' is an instance of
Char, and false an instance of Boolean. You cannot create
instances of these classes using new. This is enforced by the "trick" that value
classes are all defined to be both abstract and final. So if you were to write:

The other value class, Unit, corresponds roughly to Java's void
type; it is used as the result type of a method that does not
otherwise return an interesting result. Unit has a single instance
value, which is written (), as discussed in Section 7.2.

As explained in Chapter 5, the value classes support the usual
arithmetic and boolean operators as methods. For instance, Int
has methods named + and *, and Boolean has
methods named || and &&. Value classes also inherit all methods
from class Any. You can test this in the interpreter:

Note that the value
class space is flat; all value classes are subtypes of
scala.AnyVal, but they do not subclass each other. Instead there
are implicit conversions between different
value class types. For example, an instance of class scala.Int is
automatically widened (by an implicit conversion) to an instance of class scala.Long when
required.

As mentioned in Section 5.9,
implicit conversions are also used to add more functionality to
value types. For instance, the type Int supports all of the operations below:

Here's how this works: The methods min, max, until, to,
and abs are all defined in a class scala.runtime.RichInt, and
there is an implicit conversion from class Int to RichInt. The
conversion is applied whenever a method is invoked on an Int
that is undefined in Int but defined in RichInt. Similar
"booster classes" and implicit conversions exist for the other value classes.
Implicit conversions will be discussed in detail in
Chapter 21.

The other subclass of the root class Any is class AnyRef. This
is the base class of all reference classes in Scala. As mentioned previously, on the
Java platform AnyRef is in fact just an alias for classjava.lang.Object. So classes written in Java as well as classes
written in Scala all inherit from AnyRef.[1]
One way to think of java.lang.Object, therefore, is as the way AnyRef is implemented on the
Java platform. Thus, although you can use Object
and AnyRef interchangeably in Scala programs on the Java platform, the recommended style
is to use AnyRef everywhere.

Scala classes are different from Java classes in that they also
inherit from a special marker trait called ScalaObject. The idea is
that the ScalaObject contains methods that the Scala compiler
defines and implements in order to make execution of Scala programs
more efficient. Right now, Scala object contains a single method, named
$tag, which is used internally to speed up pattern matching.

How is all this implemented? In fact, Scala stores integers in the
same way as Java: as 32-bit words. This is important for efficiency on
the JVM and also for interoperability with Java libraries. Standard
operations like addition or multiplication are implemented as
primitive operations. However, Scala uses the "backup" class
java.lang.Integer whenever an integer needs to be seen as a (Java) object.
This happens for instance when invoking the toString method on an integer
number or when assigning an integer to a variable of type Any.
Integers of type Int are converted transparently
to "boxed integers" of type java.lang.Integer whenever necessary.

All this sounds a lot like auto-boxing in Java 5 and it is indeed quite similar.
There's one crucial difference, though, in that boxing in Scala is much less visible
than boxing in Java. Try the following in Java:

You will find that you get false! What happens is that the number 421 gets boxed twice,
so that the arguments for x and y are two different objects.
Because == means reference equality on reference types, and Integer is a reference type,
the result is false. This is one aspect where it shows that Java is not a
pure object-oriented language. There is a difference between primitive types and reference types
that can be clearly observed.

In fact, the equality operation == in Scala is
designed to be transparent with respect to the type's
representation. For value types, it is the natural (numeric or
boolean) equality. For reference types, == is treated as
an alias of the equals method inherited from
Object. That method is originally defined as
reference equality, but is overridden by many subclasses to
implement their natural notion of equality.
This also means that in Scala
you never fall into Java's well-known trap concerning string comparisons.
In Scala, string comparison works as it should:

In Java, the result of comparing x with y would be false. The
programmer should have used equals in this case, but it is easy
to forget.

However, there are situations where you need reference equality
instead of user-defined equality. For example, in some situations where
efficiency is paramount, you would like to hash cons
with some classes and compare their instances with reference
equality.[2]
For these cases, class AnyRef
defines an additional eq method, which cannot be
overridden and is implemented as reference equality (i.e., it behaves
like == in Java for reference types). There's also the negation of eq, which is called ne.
For example:

At the bottom of the type hierarchy in Figure 11.1
you see the two classes
scala.Null and scala.Nothing. These are special types that handle some
"corner cases" of Scala's object-oriented type system in a uniform way.

Class Null is the type of the null reference; it is a subclass of
every reference class (i.e., every class that itself inherits from AnyRef).
Null is not compatible with value types.
You cannot, for example, assign a null value to an integer
variable:

Type Nothing is at the very bottom of Scala's class hierarchy; it is a subtype of every other type. However, there exist no values of
this type whatsoever. Why does it make sense to have a type without
values? As discussed in Section 7.4, one use of Nothing is that it signals abnormal
termination. For instance there's the error method in the Predef object of
Scala's standard library, which is defined like this:

The return type of error is Nothing, which tells users that
the method will not return normally (it throws an exception instead).
Because Nothing is a subtype of every other type,
you can use methods like error in very flexible ways. For instance:

The "then" branch of the conditional, x/y, has type Int, whereas the
else branch, the call to error, has type Nothing. Because Nothing is a subtype of
Int, the type of the whole conditional is Int, as required.

In this chapter we showed you the classes at the top and bottom of Scala's class hierarchy.
Now that you've gotten a good foundation on class inheritance in Scala, you're ready to understand mixin composition.
In the next chapter, you'll learn about traits.

Footnotes for Chapter 11:

[1] The reason the AnyRef alias exists, instead of just
using the name java.lang.Object, is because Scala was designed to work on both the Java
and .NET platforms. On .NET, AnyRef is an alias for System.Object.

[2] You hash
cons instances of a class by caching all instances you have created
in a weak collection. Then, any time you want a new instance of the
class, you first check the cache. If the cache already has an
element equal to the one you are about to create, you can reuse the
existing instance. As a result of this arrangement, any two
instances that are equal with equals() are also equal with reference
equality.